The present invention relates to a method in which a gas stream contaminated with per-fluorocarbons is treated by contacting the per-fluorocarbons with calcium hydride at an elevated temperature.
In semiconductor fabrication techniques of plasma assisted chemical vapor deposition, process gases are used in depositing materials on silicon wafers in one or more deposition stages. Since the deposition stages result in material being deposited on the interior of the processing chamber itself, the processing chamber must be cleaned at conclusion of the deposition stages. In order to clean the interior of the processing chamber, a per-fluorocarbon, for instance, carbon tetrafluoride (CF.sub.4) or hexafluoroethane (C.sub.2 F.sub.6) and oxygen are introduced into the processing chamber to react with materials which have been deposited on the walls of the processing chamber. In another fabrication technique known in the art as plasma etch, carbon tetrafluoride and hexafluoroethane are used to selectively remove deposited materials from the substrate. Both techniques, plasma assisted chemical vapor deposition and plasma etch, use excess amounts of such per-fluorocarbons and as a result, an exhaust gas discharged from the processing chamber will comprise per-fluorocarbons.
At various stages of the semiconductor fabrication process, per-fluorocompounds such as nitrogen trifluoride (which is used in tungsten depositions) are removed from the exhaust gas stream by a gas reactor column having silicon and calcium oxide containing stages which are effective to treat nitrogen trifluoride and other per-fluorocompounds but which are not effective in treating carbon tetrafluoride or hexafluoroethane. A gas reactor column used for such treatment is fully described in U.S. Pat. No. 5,213,767.
The use of per-fluorocarbons and per-fluorocompounds is not limited to semiconductor production. Per-fluorocarbons are produced by the Hall-Heroult process. In this process, alumina is dissolved in cryolite and then is reduced electrochemically to metallic alumina. This process occurs at about 1000.degree. C. with carbon acting as both electrodes. The per-fluorocarbons are produced as a by-product when the amount of alumina contained within the electro-chemical cell is low purity.
Although combustion and incineration techniques have been proposed to treat per-fluorocarbons, in practice, exhaust streams containing per-fluorocarbons are simply vented to the atmosphere from the semiconductor etching processes. It is to be noted that the treatment of carbon tetrafluoride produced in excimer lasers from excimer gases is known in the art. In DE 4002642, carbon tetrafluoride is removed from a laser gas by exposing such gas to an alkaline earth metal such as calcium at temperature above 450.degree. C. to produce carbon. This treatment is not effective when the gases to be treated emanate from the cleaning stage of a semiconductor etching process because not enough of the per-fluorocarbon compound will be converted. Simply stated, the cleansing process disclosed in DE 4002642 does not work when large volumes of gases are to be treated.
The problem with per-fluorocarbon release is that it is thought that per-fluorocarbons constitute an environmental hazard and a contributor to the global warming effect. Hence their exists a need to remove per-fluorocarbons from exhaust gas streams through simple and effective reaction techniques. As will be discussed, the present invention provides a method of treating a per-fluorocarbon containing gas stream which can be efficiently used and form part of a plasma enhanced vapor deposition process or a plasma etch process used in fabricating semi-conductors.